Core and coil with protective covering

ABSTRACT

Current-type instrument transformer adaptable for placement in close proximity with oil filled apparatus. The insulation of the current transformer includes a layer of a butyl rubber disposed adjacent to the magnetic core and winding structure of the current transformer. A thin layer of a polyurethane-based material is disposed over the rubber insulation to protect it from transformer oil and other elements which are chemically destructive to the rubber insulation layer.

United States Patent [191 [451 June 26, 1973 CORE AND COIL WITH PROTECTIVE COVERING [75] Inventor: Robert J. Hill, Sharon, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

221 Filed; Oct.15, 1971 211 App]. No.: 189,620

[52] U.S. Cl. 336/96, 336/205 [51] Int. Cl. H011 27/02 [58] Field of Search 336/96, 205;

174/120 R, 120 C, 120 AR Primary Examiner-Thomas .l. Kozma Attorney-A. T. Stratton [57] ABSTRACT Current-type instrument transformer adaptable for placement in close proximity with oil filled apparatus. The insulation of the current transformer includes a layer of a butyl rubber disposed adjacent to the mag netic core and winding structure of the current transformer. A thin layer of a polyurethane-based material [56] References Cited dispos (1 er the rubber insulatio to protect it f is e ov n rom UNITED STATES KATENTS transformer oil and other elements which are chemi- 2,743,308 4/1956 Bandsley 336/96 X ca"), destructive to h rubber insulation layer 2,707,205 4/1955 Abbott et al 174/120 R 3,206,542 9/1965 Dawson et a1. 174/120 4 Claims, 4 Drawing Figures IllIIIlllIIIIlIlIIIIIIIlIlIIII lllllllllllllllllllIIIlIIllIII lIIllIIIII-IIIIIIIIII IlllIjrII-IIIIIIIIII PAIENTEDmzs \sn SHEEIZUFZ FIG. 4.

5 4 I V I CORE AND COIL WITH PROTECTIVE COVERING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates, in general, to electrical inductive apparatus and, more specifically, to current-type instrument transformers.

2. Description of the Prior Art Electrical transformers, such as current-type instrument transformers, are used in a variety of applications to make it possible to measure high currents. Current transformers may be used with power transmission lines to monitor the current therein. Current transformers may also be positioned on bus conductors to measure the current carried by the bus conductors. Power distribution transformers often have current transformers mounted over the bushings of the distribution transformer to measure the transformer current.

When the current transformer is used in close proximity with oil filled apparatus, or in a highly oil contaminated atmosphere, normal insulation materials may become damaged. This problem is especially prevelant with oil-filled power distribution transformers which have current transformers mounted directly over the low voltage bushings. A slight leak in the bushing seal may cause oil to come into contact with the insulation of the current transformer. The bushings on pad mounted power distribution transformers are more susceptible to leaking due to the fact that the bushings usually extend through a vertical wall of the transformer at a position which is below the oil level.

Lowly unsaturated elastomers, such as vulcanized butyl or ethylenepropylene rubbers, have been used extensively in the prior art as the insulating material for current transformers of the type suitable for mounting on transformer bushings. Unfortunately, these elastomers do not provide adequate chemical resistance to transformer oil. The transformer oil causes the elastomeric insulation to swell upon prolonged contact with the oil.

It is desirable, and it is an object of this invention, to provide a means for economically and effectively increasing the chemical stability of elastomeric transformer insulation.

SUMMARY OF THE INVENTION There is described a new and useful current transformer which may be economically constructed of readily available materials. The magnetic core and winding assembly of the current transformer is coated with a butyl rubber layer which provides the major portion of the electrical insulation around the core and winding assembly. The rubber layer is coated with a protective coating to reduce the chemical effect of certain substances, such as transformer oil, on the rubber layer.

The protective layer may comprise a polyurethanebased material. It is applied to the rubber layer to form a relatively thin layer thereon to chemically protect the rubber layer. Application may be by either dip or spray methods, or any other suitable method.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of this invention will become more apparent when considered in view of the following detailed description and drawings, in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the following description, similar reference characters refer to similar members in all figures of the drawings.

Referring to the drawings, and FIG. 1 in particular, there is shown a power distribution transformer 10 suitable for pad mounting. The transformer 10 includes the casing 12, the high voltage compartment door 14, and the low voltage compartment door 16 which is illustrated in an open position. Low voltage phase bushings 18, 20 and 22 are mounted on the vertical casing wall 24. Current-type instrument transformers 26, 28 and 30 are mounted directly over the phase bushings 18, 20 and 22, respectively. The low voltage neutral bushing 32 is shown without a-current transformer attached thereto. The leads which would normally be connected to the bushings 18, 20, 22 and 32, and to the current transformers 26, 28 and 30, are not illustrated. The current transformers 26, 28 and 30 shown in FIG. 1 are constructed according to the teachings of this invention which are described hereafter in more detail.

FIG. 2 shows another arrangement utilizing a current transformer which is constructed according to the teachings of this invention. The current transformer 34 is constructed similar to the current transformers 26, 28 and 30 shown in FIG. I. The current transformer 34 includes the winding terminals 36 and 38. Mounting holes 40 may be used to secure the current transformer 34 to a bracket or to other types of supporting structures. The current transformer 34 is illustrated mounted around a bus conductor 42. Current flowing in the bus conductor 42 induces a current into the winding of the current of the current transformer which varies proportionally with the current in the bus conductor. By attaching a suitable instrument to the terminals 36 and 38, the current flowing in the bus conductor 42 can be measured.

FIG. 3 is an elevational view of a current transformer which is constructed according to the teachings of this invention. The current transformer consists of a magnetic core and winding assembly 44 which is shaped to provide a window opening 46. The winding terminals 36 and 38 extend through the insulation structure 48 of the current transformer and are connected to the winding of the transformer. The insulation structure 48 provides electrical and environmental protection for the magnetic core and winding assembly 44 as well as providing additional mechanical strength for the assembly.

FIG. 4 is a cross-sectional view of a portion of the current transformer taken along the line IV-IV shown in FIG. 3. The magnetic core 50, which may be of the stacked lamination type, and the winding 52 comprise the winding assembly 44 as shown in FIG. 3. The winding 52 comprises a plurality of turns of a conducting material disposed around the magnetic core 50. The

winding 52 may be concentrated in one or more portions of the magnetic core 50 or it may be disposed continuously around the entire periphery of the magnetic core 50. It is also with the contemplation of this invention that the general shape of the magnetic core and winding assembly 44, as shown in FIG. 3, may be other than square or rectangular. In addition, the corners of the magnetic core and winding assembly 44 may be shaped to adapt the structure for other mounting arrangements.

The cross-section of the insulation structure 48 comprises a first layer 54 and a second layer 56. The first layer 54 comprises a lowly unsaturated type elastomer, such as vulcanized butyl or ethylene-propylene rubber. The thickness of the first layer 54 is sufficient to provide the required electrically insulation for the current transformer. The first layer also provides some mechanical support for the winding 52 and the core 50. The thickness of the first layer 54 is highly dependent upon the rating of the current transformer. A first layer 54 having a thickness of about one-fourth inch is typical for this type of current transformer. The second layer 56 of the insulation structure 48 is substantially thinner than the first layer 54. A thickness of 1 mil would be typical, thereby making the first layer 543 about 250 times thicker than the second layer 56. However, other second layer thicknesses may be used.

An important function of the second layer 56 is to chemically protect the first layer 54 from weathering and from harmful substances, such as transformer oil. An oil resistant material from which the second layer 56 may be constructed could consist of a polyurethanebased material. A suitable material would be Chemglaze 11, which are polyurethane coatings marketed by Hughson Chemical Co., a Division of Lord Corporation. Although these are one-part moisture curable materials which can be applied by dip or spray methods, two-part materials can be successfully applied by an appropriate method.

Field conditions have been simulated in the laboratory to compare the effects of transformer oil on current transformers constructed according to this invention and on current transformers constructed according to the prior art. When exposed to dripping transformer oil, the unprotected butyl rubber insulation of a prior art current transformer exhibited swelling and deformation. A current transformer constructed according to this invention showed no signs of swelling or deformation. Coated and uncoated specimens were also emerged in transformer oil for a period of two days at a temperature of 60C. The uncoated specimen showed an insulation volume increase of over 100 percent while the coated specimen, constructed according to this invention, did not exhibit any increase in insulation volume.

There has been disclosed new and useful apparatus for improving the oil resistance of current-type instrument transformers. Since numerous changes may be made in the above described apparatus and different embodiments of this invention may be made without departing from the spirit thereof, it is intended that all of the matter contained in the foregoing description, or shown in the accompanying drawings, shall be interpreted as illustrative rather than limiting.

I claim as my invention:

1. Electrical apparatus adapted for placement adjacent to oil-filled apparatus, said electrical apparatus having a magnetic core, a winding assembly, terminal means attached to said winding assembly, said magnetic core and said winding assembly being enclosed within a first layer comprising a predetermined thickness of rubber, said first layer being enclosed within a second layer comprising a coating of a polyurethanebased material.

2. The electrical apparatus of claim 1 wherein the first layer has a thickness which is at least 50 times the thickness of the second layer.

3. The electrical apparatus of claim 1 wherein the thickness of the second layer is less than 10 mils.

41. A current-type instrument transformer adapted for placement adjacent to oil-filled apparatus, said transformer having an oil resistant cover thereon, said transformer comprising a laminated magnetic core, a winding assembly disposed on said magnetic core, said transformer having an opening therethrough for placing a current conducting member therein, terminal means attached to said winding assembly, said terminal means protruding from said oil resistant cover, said oil resistant cover comprising a first layer of a predetermined thickness of butyl rubber which is positioned adjacent to said magnetic core and said winding assembly, said oil resistant cover also comprising a second layer of a predetermined thickness of polyurethane which is positioned adjacent to said first layer, the first layer having a thickness which is at least 50 times the thickness of the second layer, and the thickness of the second layer being less than 10 mils. 

2. The electrical apparatus of claim 1 wherein the first layer has a thickness which is at least 50 times the thickness of the second layer.
 3. The electrical apparatus of claim 1 wherein the thickness of the second layer is less than 10 mils.
 4. A current-type instrument transformer adapted for placement adjacent to oil-filled apparatus, said transformer having an oil resistant cover thereon, said transformer comprising a laminated magnetic core, a winding assembly disposed on said magnetic core, said transformer having an opening therethrough for placing a current conducting member therein, terminal means attached to said winding assembly, said terminal means protruding from said Oil resistant cover, said oil resistant cover comprising a first layer of a predetermined thickness of butyl rubber which is positioned adjacent to said magnetic core and said winding assembly, said oil resistant cover also comprising a second layer of a predetermined thickness of polyurethane which is positioned adjacent to said first layer, the first layer having a thickness which is at least 50 times the thickness of the second layer, and the thickness of the second layer being less than 10 mils. 